1. Field of the Invention
The application relates to a method utilized in a wireless communication system, and more particularly, to a method for radio access technology change in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
Moreover, inter-radio access technology (inter-RAT) mobility involves transfer of a connection or mobility between a UE and a network from one RAT (e.g. the LTE system) to another RAT, such as Global System for Mobile communications (GSM)/Global Packet Radio Service (GPRS), Universal Mobile Telecommunications System (UMTS), or Code Division Multiple Access (CDMA) system. However, the applicant notices that a UE may encounter problems associated to the inter-RAT procedure as below based on a direct image on a basis of the prior art LTE system.
A UE performs the inter-RAT procedure to move to a RAT due to handover, cell redirection, or cell reselection. After that, if the UE needs to move to a previously camped RAT, the UE executes PLMN search, cell redirection or cell reselection. In the case of PLMN search, the UE has to stop all of the on-going procedures, re-searches for a PLMN, and then searches for a cell of the PLMN. However, with such manner, the UE may not be able to move to the camped RAT, or takes a lot of time to move to the camped RAT. In the case of cell reselection or cell redirection, a network (e.g. a cell, NB or eNB) of the RAT has to provide the UE of cell information or redirection information associated to the camped RAT. Thus, the UE can perform measurement according to the cell information, and thereby reselects a cell of the camped RAT if the measured cell fulfills a criterion in cell reselection, or can be directed to the camped RAT. However, these methods require signaling between the UE and the network, which increases signaling number. In addition, the network may require to be upgraded to provide abovementioned cell information/redirection information, which increases the cost to the network operator.
For example, a UE in the LTE system may fallback to legacy system, such as UMTS and GSM, due to out of service, cell reselection, handover, circuit-switched fallback (CS fallback) and Internet Protocol Multimedia Subsystem (IMS) Single Radio Voice Call Continuity (SRVCC). Once the UE fallbacks to the legacy system, it is difficult to return to LTE due to lack of LTE cell information provided by the legacy system. Note that, the legacy systems generally lack of LTE cell information due to network planning. More specifically, in order to return to the LTE system, the UE may perform PLMN search/cell reselection/cell redirection. In the case of PLMN search, the UE has to stop all of the on-going procedures, re-searches for a PLMN, and then searches for a cell of the PLMN. With such manner the UE may not be able to return to the LTE system and may take a lot of time to return to the LTE system. In the case of cell reselection, the cell of the legacy system has to provide LTE cell information to the UE (i.e. by system information broadcasting), and thereby the UE performs measurement based on the LTE cell information. If signal strength or signal quality of a measured LTE cell reaches the criterion in the cell reselection, the UE selects the LTE cell so as to return to the LTE system. However, due to the network planning, the cell of the legacy system carries no LTE cell information. Thus, in order to provide LTE cell information, cells of the legacy system have to be upgraded, which increases the cost to the network operator. In the case of the cell redirection, the network (i.e. NB) of the legacy system has to provide redirection information to indicate the UE to return to the LTE system (i.e. by a RRC connection release message), and thereby the UE returns to the LTE system. However, in order to achieve this, the network of the legacy system has to be upgraded, causing the cost increase.
In addition, a UE in higher priority system may fallback to lower priority system due to out of service, cell reselection, handover, circuit-switched fallback and IMS SRVCC. Once the UE fallbacks to lower priority system, it may be difficult to return to higher priority system due to the lack of cell information of higher priority system provided by lower priority system. Lower priority system may be lack of cell information of higher priority system due to network planning. The detailed description can be referred from above, so it is not given herein.